1. Field of the Invention
The present invention relates to an improvement of a high-power semiconductor device, such as a static induction thyristor, a gate turn-off thyristor, and the like and, more particularly, to a switching semiconductor device which is capable of high-speed switching of a large current and has a low forward voltage drop and a high blocking voltage between main electrodes.
2. Description of the Prior Art
In control of motors or application of switching power sources by a pulse-width modulation method, if a switching speed is low, a switching loss increases as a control frequency increases, and a heat dissipation arrangement for a device is complex, resulting in a bulky system. If the control frequency is decreased below an audible frequency (20 kHz or lower), a noise generated by the device makes an operator uncomfortable. If acoustic insulation is made in order to prevent this, this also results in a bulky system.
For inductance equipment, e.g., a transformer, it is known that its weight is inversely proportional to 1/2 square of a frequency. From this point of view, a decrease in control frequency results in a bulky system.
Therefore, in control of motors or application in switching power source by the pulse-width modulation method, a control frequency must be increased, and high-speed switching is required for this reason. As for losses, it is important to reduce the ON state loss as well as the switching loss. A reduction in ON state loss requires a low forward voltage drop.
In addition, a high blocking voltage of several kilovolts between main electrodes is required for the application to a high-voltage line system.
A semiconductor device, such as a static induction thyristor, gate turn-off thyristor, or the like, comprises two main electrode regions, i.e., anode and cathode regions consisting of high impurity concentration regions of opposite conductivity types, a low impurity concentration region locally formed between these two regions, and a gate region, formed near the cathode region, for controlling a main current.
In the conventional semiconductor device of this type, it is known that when the thickness of the lower-impurity concentration region is increased, a high blocking voltage between the main electrodes can be realized. In the semiconductor device, in order to realize high-speed switching, a method for uniformly reducing the carrier lifetime on the entire volume of the lower-impurity concentration region by diffusion of gold is proposed. With this method, however, if the carrier lifetime is shortened enough to realize high-speed switching, a forward voltage drop is significantly increased, and high-speed switching and a low forward voltage drop cannot be realized at the same time.
In order to solve the above problem and realize high-speed switching and low forward voltage drop at the same time, a structure wherein a region having a relatively low carrier lifetime is provided in the low impurity concentration region locally with respect to the main current direction in a static induction thyristor has been proposed.
With this structure, however, only one region having a relatively low carrier lifetime is provided. Therefore, if the low impurity concentration region is formed thick in order to realize a high blocking voltage between the main electrodes, sufficiently high-speed switching and sufficiently low forward voltage drop cannot be realized at the same time.